The present disclosure relates, in various exemplary embodiments, to organic light emitting devices (OLEDs). In particular, the present disclosure relates to stacked OLED configurations.
Organic light emitting devices (OLEDs) represent a promising technology for display applications. A typical organic light emitting device includes a first electrode; a luminescent region comprising one or more electroluminescent organic material(s); and a second electrode; wherein one of the first electrode and the second electrode functions as a hole injecting anode, and the other electrode functions as an electron injecting cathode; and wherein one of the first electrode and the second electrode is a front electrode, and the other electrode is a back electrode. The front electrode is transparent (or at least partially transparent) while the back electrode is usually highly reflective to light. When a voltage is applied across the first and second electrodes, light is emitted from the luminescent region and through the transparent front electrode.
It is sometimes desirable to laminate two or more individual OLEDs in a stacked configuration to form a stacked OLED. Stacked OLED configurations include intermediate electrodes disposed between adjacent luminescent regions. That is, the stacked OLED includes a plurality of individual OLEDs which are defined by a luminescent region disposed between two electrodes. A top electrode of one individual OLED in the stack also functions as a bottom electrode of another OLED in the stack. Successive luminescent regions share an intermediate electrode.
Stacked OLEDs may emit different colors such that a true color pixel is formed from which any color may be emitted. For example, Burrows et al. disclose in Appl. Phys. Lett. 69, 2959 (1996) individual OLEDs with red, green, or blue emissions stacked to form color-tunable vertically integrated pixels.
Stacked, monochromatic OLEDs are also possible such as demonstrated by Matsumoto et al. (SID 03 Digest, 979 (2003)). Stacked, monochromatic OLEDs potentially provide an OLED configuration having high electroluminescence efficiency.
The intermediate electrodes are typically transparent. Additionally, the intermediate electrodes are often required to act as electron injecting contacts on one side and as hole injecting contacts on the other side. To exhibit all the features required in the intermediate electrodes, the intermediate electrodes are typically made of multiple layers and from materials, e.g., ITO, V2O5, that are hard to deposit via thermal vapor deposition processes. Consequently, fabricating stacked OLEDs requires additional, more aggressive techniques, such as, for example, sputtering. The required additional, more aggressive deposition techniques increases the costs of fabricating the OLEDs and also increases the risk of damaging the other more fragile organic layers of the stacked OLED.
Therefore, there is a need to provide an intermediate electrode composition or configuration that allows for a stacked OLED to exhibit the advantages of existing stacked OLED configurations. There is also a need to provide an intermediate electrode having a light absorbing capability. There is also a need to provide an intermediate electrode composition or configuration suitable for use in a stacked OLED that is amenable to physical vapor deposition processing.